1. Field of the Invention
The present invention relates to buckling restrained braces used in buildings and steel structures and capable of absorbing vibration energy generated by an earthquake, wind power, etc.
2. Description of the Related Art
Japanese Examined Utility Model (Kokoku) No. 4-19121 discloses a buckling-constraining brace member in which an adhesion-preventive film is provided between a center axial member and a concrete member. Japanese Unexamined Utility Model (Kokai) No. 5-3402 discloses a buckling-constraining brace member wherein a steel-made center axial member is passed through a steel-made buckling-constraining member, and an adhesion-preventive film is placed between the surface of the center axial member and the buckling-constraining member. Japanese Unexamined Utility Model (Kokai) No. 5-57110 discloses a damping brace member wherein both ends of an intermediate member having a small cross section are each connectively and integrally jointed to one end of a side member having a large cross section, in series to form a steel-made center axial member, and the axial member is fitted in a constituent hollow buckling-constraining member. Japanese Unexamined Utility Model (Kokai) No. 5-57111 discloses a damping brace member having the same constitution as in Japanese Unexamined Utility Model (Kokai) No. 5-57110 and excellent in damping properties, durability and weatherability. Japanese Unexamined Patent Publication (Kokai) No. 7-229204 discloses that the stiffness and yield stress of a buckling-constraining brace member can be arbitrarily determined, and that the stress flow of the steel-made center axial member is improved. R. Tremblay et al. reported experimental result relate to buckling-constraining members in the 8th Canadian conference on Earthquake Engineering (cf. Seismic Rehabilitation of a Four-stored Building with a Stiffened Bracing System, published on, Jan. 19, 1999).
An adhesion-preventive film is provided between a buckling-constraining concrete member reinforced with a steel material and a steel-made center axial member for the purpose of preventing the steel-made center axial member from adhering to the concrete of the buckling-constraining concrete member. The following problems, about the adhesion-preventive film, arise. When the adhesion-preventive film is too thin, the film does not tolerate the expansion in the plate thickness direction of the steel-made center axial member caused by its axial deformation; on the other hand, when the adhesion-preventive film is too thick, it is incapable of constraining local buckling of the steel-made center axial member. Moreover the adhesion-preventive film has still other problems as mentioned below. When the stiffness in the thickness direction of the adhesion-preventive film is too low, it is incapable of maintaining a predetermined thickness due to the concrete pressure during pouring concrete; moreover, when the stiffness in the thickness direction thereof is too high, it cannot absorb the expansion in the plate thickness direction of the steel-made center axial member caused by the influence of Poisson""s ratio at the time of plasticization, namely, plastic deformation of the steel-made center axial member.
When a plain steel (yield stress "sgr"y=235 N/mm2) is used for the steel-made center axial member of a buckling restrained brace, there arises a problem that the buckling restrained brace cannot be made to function as a hysteresis damper against an earthquake of a small magnitude because the steel-made center axial member does not yield at the early stage against a ground motion acceleration (80 to 100 gal) of the earthquake.
A steel-made center axial member of a buckling restrained brace having the same cross-sectional area from one end of the member, through the central portion, to the other end has the following problem. When the steel-made center axial member is made to function as a hysteresis damper, both ends as well as the central portion of the member are plasticized (plastically deformed) due to yielding, and consequently fracture at joints between the buckling restrained brace and a steel structure including a column and a beam takes place.
In the process of producing a buckling-constraining concrete member of a buckling restrained brace reinforced with a steel material, when the ends of the reinforcing steel material of a buckling-constraining concrete member are open, there arise problems as mentioned below. During pouring the concrete, the concrete flows out before its solidification, and pouring concrete becomes difficult; cracked concrete falls during the use of the buckling restrained brace. Furthermore, an adhesion-preventive film is placed between the buckling-constraining concrete member of the buckling restrained brace reinforced with the steel material and the steel-made center axial member for the purpose of preventing mutual adhesion between the axial member and the concrete member. Accordingly, the following problem arises. When the steel-made center axial member is axially deformed due to vibration generated by an earthquake or wind power, it is not definite in which of two directions, a direction towards one end of the steel-made center axial member and a direction towards the other end thereof, the buckling-constraining concrete member is moved, and the concrete member is deflected to one of the two ends when the concrete member starts to be moved.
When the buckling restrained brace is to be mounted on a damping steel structure, the buckling restrained brace is generally jointed with high tensile bolts. In jointing the buckling restrained brace, the following problem arises. When the axial tension of the steel-made center axial member increases, the number of bolts used significantly increases, and the buckling restrained brace cannot be fixing jointed unless both of its ends are extremely expanded. Moreover, the width of both ends of the buckling restrained brace cannot be increased much because the width is restricted by the widths of columns and beams of the damping steel structure on which the buckling restrained brace is to be mounted.
The buckling restrained brace has a problem that the steel-made center axial member cannot be made to function as a hysteresis damper for absorbing vibration energy of the micro-vibration of an earthquake of very small magnitude, wind power, etc., to which the steel-made center axial member does not yield.
When the steel structure is shaken by an earthquake of a large magnitude, part of the columns, beams and braces of the steel structure are plasticized. Even when they are plasticized, the steel structure does not collapse so long as they have a sufficient capacity of plastic deformation and sufficient resistant to fatigue. However, jointed portions and welded portions prepared by field fabrication tend to decline in quality compared with those prepared by factory production, and are sometimes fractured before performing a sufficient plastic deformation function. When these columns, beams and braces are plasticized, the steel structure is deformed, and there arises a problem that the steel structure must be repaired on a large scale if it is to be used after the earthquake.
The problems mentioned above are solved by a buckling restrained brace 1 according to the present invention wherein a steel-made center axial member 3 is passed through a buckling-constraining concrete member 2 reinforced with a steel member 6, and an adhesion-preventive film 4 is provided to the interface between the steel-made center axial member and buckling-constraining concrete 5, the adhesion-preventive film showing a secant modulus in the thickness direction of at least 0.1 N/mm2 between a point which shows a compressive strain of 0% and a point which shows a compressive strain of 50%, and up to 21,000 N/mm2 between a point which shows a compressive strain of 50% and a point which shows a compressive strain of 75%, and having a thickness dt in the plate thickness direction of the steel-made center axial member 3 and a thickness dw in the plate width direction thereof from at least 0.5 to 10% of the plate thickness t and from at least 0.5 to 10% of the plate width w, respectively.
When considering pressure for placing concrete 5 in manufacturing a buckling-restraining brace 1, a desirable minimum thickness ratio of the adhesion-preventive film 4 and a steel-made center axial member 3 is preferably in the range from not less than 1.2% to up to 10%.
Moreover, in the buckling restrained brace according to the present invention, the steel-made center axial member 3 is a steel material showing a 0.2% proof stress or a yield point stress of up to 130 N/mm2.
Furthermore, in the buckling restrained brace according to the present invention, the steel-made center axial member 3 is a steel material showing a 0.2% proof stress or a yield point stress of 130 to 245 N/mm2.
Still furthermore, in the buckling restrained brace according to the present invention, the steel-made center axial member 3 has a minimum cross-sectional area in a central portion 21 in the longitudinal direction having a restricted length ratio which is the ratio of the length of the central portion to the whole length, and the steel-made center axial member has a cross-sectional area larger than the minimum cross-sectional area of the central portion 21 in the longitudinal direction, at both ends 22, 23 in the longitudinal direction connectively provided to the central portion in the longitudinal direction.
Moreover, in the buckling restrained brace 1 having a cross-sectional area of the central portion (21) as described in the above, the steel-made center axial member (3) shows an axial equivalent stiffness of at least 1.5 times that of the steel-made center axial member (3) which has same-sectional area from one end to the other end, passing through the central portion (21) in the length direction of said member (3).
Furthermore, in the buckling restrained brace according to the present invention, each of the cross-sectional areas 22-1, 23-1 at both ends 22, 23 in the longitudinal direction of the steel-made center axial member 3 which is obtained by subtracting a through hole-formed deficient area of the corresponding through holes for bolt insertion passing is at least 1.2 times the cross-sectional area 21-1 of the central portion 21 in the longitudinal direction of the steel-made center axial member.
Moreover, in the buckling restrained brace 1 according to the present invention, the steel member 6 is a reinforcing bar 6-1.
Still furthermore, in the buckling restrained brace 1 according to the present invention, a lid 24 is fixed to at least one end of the buckling-constraining concrete member 2.
Moreover, in the buckling restrained brace according to the present invention, a slip stopper 25 is provided to the center of the steel-made center axial member 3.
Furthermore, in the buckling restrained brace 1 according to the present invention, the buckling restrained brace 1 having the steel-made center axial member 3 which is provided with through holes 26 for bolt insertion at both ends 22, 23, and steel-made connecting plates 27 are friction jointed with high tension bolts by clamping, while the friction face sides at both ends 22, 23 of the steel-made center axial member which are contacted with the respective friction face sides of the steel-made connecting plates 27 or the friction face sides of the steel-made connecting plates 27 which are contacted with the respective friction face sides at both ends 22, 23 of the steel-made center axial member are made to have a higher surface hardness and a higher surface roughness than the counterpart friction face sides.
Still furthermore, in the buckling restrained brace according to the present invention, at least one set, comprising three layers which are formed from a C-shaped cross-sectional inside steel plate 29, a visco-elastic sheet 30 and a C-shaped cross-sectional outside steel plate 31, is fastened to each of the sides of the buckling-constraining concrete member 2 of the buckling restrained brace 1; one end 32 of the C-shaped cross-sectional inside steel plate 29 is fastened to one end 34 of the buckling restrained brace 1; and the other end 33 of the C-shaped cross-sectional outside steel plate 31 is fastened to the other end 35 of the buckling restrained brace 1 in the direction opposite to the one end 32 of the C-shaped cross-sectional outside steel plate 29.
Still furthermore, the problems mentioned above are solved by a damping steel structure 38 according to the present invention wherein the above-mentioned buckling restrained braces 1 according to the present invention are placed in the damping steel structure 38 which is formed with columns 36 and beams 37 prepared from a steel material showing a yield point stress higher than that of the steel-made center axial members 3 of the buckling restrained braces 1, the buckling restrained braces 1 showing both elastic and plastic behavior when the damping steel structure 38 vibrates under vibration action, and the steel structure 38 which is formed with the columns and the beams, showing elastic behavior.